The present invention relates to electric power plants having one or more gas turbines and more particularly to systems employed in such plants for controlling the emission of nitrogen oxide in the gas turbine exhaust.
Generally, the products of combustion of oil or natural gas include nitrogen oxide which is an undesirable pollutant. Thus, thermal nitrogen oxide (NOx) is formed from nitrogen in the air in or near the combustion zone. Organic NOx is formed during combustion from nitrogen bound in the fuel and accordingly is primarily a function of the fuel nitrogen content.
The amount of thermal NOx generated by the turbine depends on variables including the type of fuel, the air/fuel ratio and the combustor design. Since thermal NOx formation is also a function of flame temperature, it can be reduced by injecting a heat sink such as water or steam into the combustion zone. Various prior art patents have dealt with this subject.
In De Corso U.S. Pat. No. 3,826,080, for example, there is disclosed a mechanical system for injecting water into the combustors of a gas turbine. However, little specific disclosure is provided on the way in which the injection system operates.
As another example, Kydd U.S. Pat. No. 3,693,347 discloses a steam injection system which provides increased cycle efficiency by maximizing steam flow and increasing turbine mass gas flow. Maximum allowed steam flow is adjusted for high and low ambient temperatures.
None of the known prior art discloses a system in which coolant injection is controlled to provide only needed reductions in NOx production on the basis of the condition of predetermined variables. Thus, where demineralized water is used as the coolant, for example, the cost of the coolant itself can be prohibitive unless the coolant is used only according to need. Further, the independent injection of coolant into the combustion zone reduces combustion efficiency and increases mass flow to reduce cycle efficiency. Therefore it is desirable for efficiency reasons to provide only the coolant injection needed for reduction of NOx to the acceptable level, i.e. the national standard defined by the U.S. Environmental Protection Agency or a more restrictive state standard such as Rule 67 applicable in California.